Beverage preparation machines are well known in the food science and consumer goods area. Such machines allow a consumer to prepare at home a given type of beverage, for instance a coffee-based beverage, e.g. an espresso or a brew-like coffee cup.
Today, most beverage preparation machines for in-home beverage preparation comprise a system made of a machine which can accommodate portioned ingredients for the preparation of the beverage. Such portions can be soft pods or pads, or sachets, but more and more systems use semi-rigid or rigid portions such as rigid pods or capsules. In the following, it will be considered that the beverage machine of the invention is a beverage preparation machine working a rigid capsule.
The machine comprises a receptacle for accommodating said capsule and a fluid injection system for injecting a fluid, preferably water, under pressure into said capsule. Water injected under pressure in the capsule, for the preparation of a coffee beverage according to the present invention, is preferably hot, that is to say at a temperature above 70° C. However, in some particular instances, it might also be at ambient temperature. The pressure inside the capsule chamber during extraction and/or dissolution of the capsule contents is typically about 1 to 6 bar for dissolution products, 2 to 12 bar for extraction of roast and ground coffee. Such a preparation process differs a lot from the so-called “brewing” process of beverage preparation—particularly for tea and coffee, in that brewing involves a long time of infusion of the ingredient by a fluid (e.g. hot water), whereas the beverage preparation process allows a consumer to prepare a beverage, for instance coffee within a few seconds.
The principle of extracting and/or dissolving the contents of a closed capsule under pressure is known and consists typically of confining the capsule in a receptacle of a machine, injecting a quantity of pressurized water into the capsule, generally after piercing a face of the capsule with a piercing injection element such as a fluid injection needle mounted on the machine, so as to create a pressurized environment inside the capsule either to extract the substance or dissolve it, and then release the extracted substance or the dissolved substance through the capsule. Capsules allowing the application of this principle have already been described for example in applicant's European patent n° EP 1 472 156 B1, and in EP 1 784 344 B1.
Machines allowing the application of this principle have already been described for example in patents CH 605 293 and EP 242 556. According to these documents, the machine comprises a receptacle for the capsule and a perforation and injection element made in the form of a hollow needle comprising in its distal region one or more liquid injection orifices. The needle has a dual function in that it opens the top portion of the capsule on the one hand, and that it forms the water inlet channel into the capsule on the other hand.
The machine further comprises a fluid tank—in most cases this fluid is water—for storing the fluid that is used to dissolve and/or infuse and/or extract under pressure the ingredient(s) contained in the capsule. The machine comprises a heating element such as a boiler or a heat exchanger, which is able to warm up the water used therein to working temperatures (classically temperatures up to 80-90° C.). Finally, the machine comprises a pump element for circulating the water from the tank to the capsule, optionally though the heating element. The way the water circulates within the machine is selected via a selecting valve means, such as for instance a peristaltic valve of the type described in applicant's European patent application EP 2162653 A1.
When the beverage to be prepared is coffee, one interesting way to prepare the said coffee is to provide the consumer with a capsule containing roast and ground coffee powder, which is to be extracted with hot water injected therein.
Capsules have been developed for such an application, which are described and claimed in applicant's European patent EP 1 784 344 B1, or in European patent application EP 2 062 831.
In short, such capsules comprise typically:                a hollow body and an injection wall which is impermeable to liquids and to air and which is attached to the body and adapted to be punctured by e.g. an injection needle of the machine,        a chamber containing a bed of roast and ground coffee to be extracted,        an aluminum membrane disposed at the bottom end of the capsule, closing the said capsule, for retaining the internal pressure in the chamber, the said membrane being associated with piercing means for piercing dispensing holes in the said aluminum membrane when said internal pressure inside the chamber reaches a certain pre-determined value,        optionally, means configured to break the jet of fluid so as to reduce the speed of the jet of fluid injected into the capsule and distribute the fluid across the bed of substance at a reduced speed. It is critical for the user to know when the water level in the machine tank is too low to prepare a full beverage.        
GB 2140286 A is a British application to Alfredo Cavalli published Nov. 28, 1984. It discloses a beverage preparation machine comprising a water tank. The machine comprises a switch and a spring-mounted pivot lever mechanism which is linked between the tank and the switch. When the tank contains sufficient water, the tank presses onto one end of the sprung lever mechanism, and the opposite end of the lever presses onto the switch. When the tank is empty, or at least does not contain sufficient water, it is sufficiently light to be lifted by the sprung lever mechanism, and the lever end in contact with the switch is moved away from said switch so that the machine sends a signal to the user of the machine, for warning the user that a tank refill is needed.
This existing solution is quite expensive and complex: it uses a spring-mounted lever mechanism that can break after a certain number of usage cycles, so that the beverage machine may require maintenance. This requirement is particularly important as Moreover, the spring-mounted lever is expensive as it requires specific parts which are manufactured separately, and then assembled.
Other systems were developed in the art, to avoid the above mentioned problems. Such systems use sensors which measure the water level through ultrasonic detection or through a floating magnet inside the water tank and a sensor in the lower part of the tank, which detects when said magnet is moved down sufficiently as water is pumped from the tank.
Such systems however involve electronic parts which are costly and may also break so that they require maintenance. Additionally, such magnetic or ultrasonic sensors require energy to function, which is undesirable, as it increases the overall energy consumption of the machine.
Having regard to the existing art, it is an objective of the present invention to provide the consumers with a beverage preparation machine which is able to detect when its water tank is not sufficiently filled, while involving a detection system which is as simple, cheap, and reliable as possible.